Holey silicon keeps electronic devices cool

The ongoing downsizing of integrated circuits, as a result of electronics miniaturisation, is challenging engineers to come up with new ways to thwart component overheating. Now, US researchers have revealed that cooling can be better facilitated with the help of holey silicon — a computer chip wafer with tiny, vertically etched orifices that work to shuttle heat to desired locations.

As explained by Jaeho Lee, from the University of California, Irvine’s (UCI) Nano Thermal Energy Research Group, the temperature problem in electronics has grown in the past few years as microchip designers seem to have reached a size boundary. With larger components, manufacturers can use heat sinks, fins and even fans to funnel warmth away from critical hardware. On today’s densely packed chips with billions of nanoscale transistors — often sandwiched in slim, pocketable consumer products — there’s no room for such cooling technologies.

Other key issues are longevity and reliability. Semiconductor chips are being embedded in many new places, acting as sensors and actuators in cars and appliances and as nodes along the Internet of Things. These devices are expected to run continuously for years and even decades, but prolonged exposure to heat could cause the failure of such infrastructure.

“It’s important that we continue to develop a better understanding of the fundamentals of thermal transport and find ways to control heat transfer at the nanoscale,” said Lee.

In 2017, Lee and his collaborators at UC Berkeley employed nanometre-scale silicon mesh material to investigate properties of phonons — quasiparticles that give scientists insight into thermal transport mechanisms. The results were published in the journal Nature Communications.

“We know that phonons can show wave-like as well as particle-like behaviour during thermal transport,” Lee said. “Using meshes with different hole sizes and spacing, we were able to clarify complex thermal transport mechanisms at the nanoscale.”

Knowledge gained from this study helped Lee understand how small, neck-shaped structures created by the etched holes in holey silicon cause phonon backscattering — a particle effect leading to low in-plane thermal conductivity. High cross-plane thermal conductivity was caused by long-wavelength phonons that help to move heat away.

Now, Lee is corresponding author on a new study published in the journal Nanotechnology. He revealed, “We found that heat prefers to travel vertically through but not laterally across holey silicon, which means the material can effectively move the heat from local hot spots to on-chip cooling systems in the vertical direction while sustaining the necessary temperature gradient for thermoelectric junctions in the lateral direction.”

The lead author on the study, graduate student Zongqing Ren, said that lab simulations demonstrated that the cooling effectiveness of holey silicon is at least 400% better than chalcogenides — compounds commonly used in thermoelectric cooling devices. He thus concluded, “This innovation could potentially be ideal for keeping electronic devices such as smartphones cool during operation.”

Image caption: Jaeho Lee, UCI assistant professor of mechanical & aerospace engineering, believes that holey silicon might be a breakthrough in the quest to keep modern electronics cool. Image credit: Steve Zylius/UCI.